Bowed Flooring Panels

A moisture imbalance is the likely cause of movement in wood-veneer flooring panels. June 11, 2014

Question (WOODWEB Member) :
We are a small factory in India and we manufacturer engineered wooden flooring. We paste 5 mm thick teak wood pieces of 900 mm length and 120 mm width on 12 mm thick pine plywood. The peices are pasted together with EPI Glue System on a temperature of 100 degrees from the top for approximately six minutes. The problem is that when the pieces are taken out from the hot press they are completely straight, but they bow after some time. We try to keep the pieces as straight as possible for storing. Can someone help us on the issue?

Forum Responses(Veneer Forum)
From contributor J:Just to get a potential language barrier out of the way, do they bow (curve lengthwise) or do they cup (bending across the 120mm width)?

From contributor E:It sound like you are making an unbalanced panel – make sure to balance the panel.
From the original questioner:
The panels bow lengthwise.
From contributor J:Your problem is beyond my expertise. It's not surprising to me that a plywood panel with a very thick veneer of solid wood glued to only one side would be problematic, but I don't know enough to point to the exact source of the lengthwise movement.
From Gene Wengert, forum technical advisor:It is a moisture issue, 100%. It is likely that the top face is moving differently than the bottom face. Without more info, it is hard to be more precise.

I agree with Contributor E. If a panel is not balanced, the mechanical effects of different thicknesses, species or grain direction will pull the panel at different rates and the result is cupping, bowing or warping. Moisture is important, but balance is also. You can make an unbalanced panel using epoxy (no moisture) and that will warp also.
From Gene Wengert, forum technical advisor:Wood will not change size or shape unless its moisture changes. So, an unbalanced panel is not a problem until the moisture changes. As veneer is often cut with a slope of grain due to taper in the log, veneer will also tend to twist when the moisture changes. This causes the panels to warp.

Gene: I agree. At the Smithsonian (Freer Gallery for example) the environment is always at the same humidity level so wood sculptures never move once they acclimate. However, for most of us, our products are subjected to random swings in M/C. In fact, a panel that is properly balanced should expand and contract with moisture changes and not warp.

One of our customers buys grand piano tops from us. They are 5-ply basswood core layups and they have to be flat. Once they receive them they put them into a high moisture chamber for weeks, followed by a low moisture chamber to temper them and allow the different materials to attain an equilibrium, essentially relieving the internal stresses. The point being, that if they were not balanced, there is no way they could survive that kind of process.

From contributor V:As has been referenced you need to balance the panel. Also check the moisture content on the pine plywood.

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